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Borland, 5-foot-11 and 248 pounds, shook it off, to the extent you can shake off head trauma, and had a successful rookie season. He had 108 tackles and two interceptions; both interceptions were in the same game, against Eli Manning.

He didn’t start until Week 7 when he replaced star linebacker Patrick Willis, who was injured. Borland’s season ended Dec. 20, when he was placed on injured reserve with an ankle injury.

He squeezed in very good work in the limited time he had. When the rapidly changing 49ers looked at the future of their defense, one of the faces they saw was his.

How many of us would, under the same circumstances, walk away from football? This is the sport that has long separated Borland from the rest of the human race. If he was careful and wise, he could have had money for life by the time he retired.

I don’t know him, but I know he loved the game. How do I know? Every successful football player I’ve talked to, and if you make an NFL roster you certainly have enjoyed some success, loves the game.

But we get one brain with which to go through life. Damage it, and there’s nobody coming in from the sideline to serve as a substitute. Head trauma horror stories abound, and many feature NFL veterans. The next happy ending to those stories will be the first.

I suffered a concussion in late September, sustained a lot of internal bleeding and missed 21/2 months of work. I wanted to come back sooner, but my doctors are smarter than I am and they wouldn’t let me.

Some of the ramifications linger. After sitting for, say, 20 minutes, it behooves me to stand up slowly. It behooves me to hold onto something when I do. I need no more than five seconds. By then I have equilibrium. But sometimes I forget. Sometimes I choose to forget. I ought to be normal by now.

I parked in my driveway two weeks ago, got out and, instead of pausing, immediately took a step, and then another. The ground began to spin and I fell backward onto the concrete. I got my arms down behind me to break the fall. I landed hard, but my head never touched the ground.

I could control the impact. Borland can’t. Watch his Carolina counterparts Thomas Davis, Luke Kuechly and A.J. Klein.

A linebacker can’t go onto the field worried about head trauma. Davis can’t worry about the right knee he has torn up three times. They trust their instincts and do what they always have. They play.

Borland decided he couldn’t take that risk. One minor concussion (if there is such a thing as a minor concussion) is alarming. A second could change his life.